The overall goal of this project is to address the problem of premature osteoarthritis of the knee in patients following anterior cruciate ligament (ACL) injury by conducting a prospective study to fill critical gaps in the available knowledge on the nature of walking mechanics changes, graft function and cartilage thinning following ACL reconstruction. This work is important since ACL loss often leads to premature degenerative arthritis of the knee even when the ACL is reconstructed and there is increasing evidence that there are kinematic and kinetic changes at the knee during walking following ACL injury or reconstruction that influence cartilage changes over time. Preliminary work for this project has demonstrated that changes in tibiofemoral motion of the ACL deficient knee during walking were associated with cartilage thinning in the tibiofemoral joint and a significant relationship between graft placement and ambulatory mechanics was observed following ACL reconstruction. Patients operated within 3 months following ACL injury will be studied at 2 years following reconstruction (to ensure remodeling is stable) and at 4 years after surgery. A matched control population will also be studied over the same interval. The study will address the following specific aims: 1) To determine if ambulatory kinematics in patients with ACL reconstruction are influenced by graft placement. 2.) To determine if there are detectable thickness changes in the weight bearing regions of tibiofemoral cartilage over a 2 year period following ACL reconstruction that are greater than a matched healthy control group 3) To determine if specific characteristics of walking mechanics of the ACL reconstructed knees remain unchanged, or change no more than matched healthy knees, between 2 and 4 years after reconstruction for clinically successful grafts. This study will use a combination of in vivo gait analysis and 3.0T magnetic resonance imaging. Using algorithms developed by our group, we can estimate the 6 degree-of-freedom motion of the tibiofemoral joint including internal tibial rotation and anterior tibial translation. Custom algorithms also allow for the high- resolution measurement of cartilage thickness across weight-bearing regions of the cartilage. This project is important clinically since the information generated from this study will helps to isolate the some critical factors that can influence premature osteoarthritis following ACL reconstruction and thus can be used to inform improved methods for treatment planning, reconstruction and rehabilitation.